By day, Don Parker is an anesthesiologist at a local hospital. By night, he is an amateur astronomer of international renown -- another stargazer hooked on outer space as a youngster by reading Ray Bradbury -- who has contributed to some front-rank research on the planets.

He is also one of about a thousand serious amateurs around the world logging observations for the International Halley Watch network -- the largest cooperative research project in the history of astronomy.

The fabled Halley's, of course, careens by once every 76 years on its long swing around the sun. First sighted on this swing by professional astronomers in 1982, it will pass closest to Earth during the second week of April.

The last time the comet sped by, in 1910, scientists failed to get much of an international cooperative effort off the ground. This time it's different. The amateurs around the world are backing up a corps of 1,000 professional astronomers from 55 countries, mostly from the United States and Western Europe.

``The principle behind the International Halley Watch is that it's always a nice day somewhere,'' says Jim Wilson, spokesman for the network in Pasadena, Calif. So although the professionals in southern California had barely a peep at Halley through the clouds in early March, Don Parker has already snapped some photos of the comet and its blue-streaked tail, seen as clearly as a snowball across the dinner table.

He is now beginning the more serious work of taking detailed pictures for studying the comet's nucleus.

Amateurs such as Parker now have more sophisticated equipment than did the professionals in 1910. In fact, now that Parker is hooked up to a computer -- on a card table in his bedroom -- he is outfitted like a small university of 10 years ago.

But the old ways still serve, too. Amateurs from China and Romania have sent in precise, hand-drawn charts logging the comet's sky position.

Like many amateurs, Parker has had no formal training in astronomy. But he grew up in the 1940s on Ray Bradbury's ``Martian Chronicles,'' Isaac Asimov stories, and Buck Rogers films.

At 12, he made his first telescope. At 16, he built a reflector telescope, grinding his own 8-inch mirrors. As an adult, his consuming interest is in Mars and its climate.

He is co-author of some 30 articles in scientific journals and has taken some valuable photographs for professional astronomers of Mars' receding polar cap and of the planet's changing light and dark features.

He still makes his own telescopes, both because he can't afford commercial equipment and because he can achieve higher quality, he says, building it himself. A friend now makes the mirrors and optical glass.

Watching Halley's comet is a minor diversion for Parker, who photographs Mars and Jupiter nightly when they appear. He works in league with networks of both amateurs and professionals around the world.

So when he spotted dust storms on Mars in 1984, he wired colleagues in Japan, England, and Africa to watch the planet as it rotated out of view from Florida.

Typically, amateur observers such as Parker work under the guidance of professionals, who analyze the photos and observations and tell the amateurs where to look.

``They know exactly what they want, and they don't tell us,'' says Parker, explaining that the scientists don't want to bias those who are observing.

What the serious amateurs provide are searching sets of eyes spread over the globe. While professionals at research institutions have to sign up well in advance for limited time on telescopes, telescope hot-rodders such as Parker can observe every night for as long as they want.

The other side, of course, is that not only do amateurs have to buy their equipment and pay for all their own film, but they also must work for a living. Parker has his system organized well enough that he can do his measuring and photographing in half an hour a night. But April and May are the most difficult months, he says, because his observing time falls in the hours after midnight, meaning he must sleep, wake up, and go back to sleep.

The most grueling work he does is in the darkroom. He now sends his Jupiter pictures to 12 research groups, all at his own expense.

Often, he must depend on professionals to confirm what he finds in his photos. Last July, when he saw white splotches on pictures he took of Jupiter's belt, he first thought it was a flaw in the film. But after discussing it with some leading Jovian researchers, he realized it was a new disturbance of some kind on the planet.

``Virtually all the discoveries I have made, the group has made, have been accidental,'' he says. He recently formed a foundation of other serious astronomers, including four professionals, so that members could at least write their expenses off their income taxes.

Parker went into medicine as a student because it was a competing interest that stood him a better chance of earning a living.

Now he thinks about getting his doctorate in astronomy when he retires. But in the meantime, he ``jealously'' guards his amateur status.

``Once you become a professional, then you have to publish,'' he says, ``and it's not fun anymore.''